1. Field of the Invention
This invention relates methods and apparatus for controlling the input to 3-phase alternating current (AC) induction motors. More particularly it relates to methods and apparatus for separately controlling the power being supplied through each of the three input lines to such a motor, using dedicated circuits to calculate the measure of reactive power and adjust the time during which solid state relays connected in series in each of the lines are maintained in a non-conducting state.
2. Description of the Related Art
It is well known that power is wasted when an alternating current (AC) induction motor is operated under a load below its rated load, or under a varying load. It is also well know that inherent in the operation of a three-phase alternating current induction motor is the presence of reactive power, expressed as voltamps reactive (VAR), the measure of VAR related to the operational inefficiency of the motor. The greater the VAR, the greater the loss of power in the system.
The presence of VAR is caused in part by the natural inductance of the motor windings, and the interaction of motor rotor and windings, which effects tend to exert a counter force to the flow of power to the system. Power in turn is the product of the instantaneous current (measured in amperes) times the instantaneous voltage (measured in volts) traveling in a wire connected to a motor. When voltage and current values are plotted on a point by point basis, with, for example, a vertical axis Y representing magnitude and a horizontal axis X representing time, separate waveforms are generated for both voltage and current. Generally sinusoidal, over a full current cycle, the separate wave forms for current and voltage during one half cycle will be on the positive side of X axis, and during the other half cycle will be on the negative side of the X axis. These same waveforms may also contain multiple harmonics. In the case of the three-phase motor when current and voltage are plotted together, because of the inductance, and generated voltamps reactance of the motor resulting from the movement of the rotor relative to the motor windings, this inherent “backpressure” causes the current waveform to lag the voltage waveform.
When the voltage and current values are taken from their respective waveforms and multiplied on a point by point basis along the X axis, a voltamp (VA) waveform is generated. When plotted along the X and Y axis, the VA waveform will generally have mostly positive values, and to a lesser extent negative values with reference to the X axis. During that period of operation where the VA waveform takes on a negative value and lies below the X axis, there is a loss of power being sent to the motor. It is this negative part of the waveform which lies below the X axis which is a measure of the voltamps reactive (VAR).
Previous attempts to increase the operating efficiency of AC induction motors are described in various U.S. patents, including U.S. Pat. Nos. 4,439,718 and 4,052,648 to Nola, U.S. Pat. No. 4,379,258 to Sugimoto and U.S. Pat. No. 5,134,356 to El Sharkawi et al.
Another approach is described in U.S. Pat. No. 6,737,827 to Cashatt, this patent commonly assigned to Enviro World Systems, Inc., and incorporated herein by reference. In Cashatt, a method and apparatus is disclosed to control the input to both single and three phase motors using a single microprocessor to manage the control circuits and calculate the time when the conducting state of in-line, solid state relays are to be changed from non-conducting to conducting. By controlling the timing of the change, the inventor therein was able to bring the voltage and current waveforms closer together, to thus reduce the amount of VAR generated, and thereby increase motor efficiency.
In practice, in the case of a three phase system it has been found that the calculations required to be performed by the microprocessor is substantial, and thus the operational speed by which calculations can be conducted for each of the three input lines limits the frequency by which the solid state relays (SSRs) can be switched on and off. In addition, the sum of the instantaneous current and voltage values in each of the three lines may vary, and thus inaccuracies in calculated values can result when these variations are not taken into account. Accordingly, there remains a need to improve the degree of control in the switching of the SSRs from the non-conducting to the conducting state in order to improve the operational efficiency of the motor being managed.